1. Field of the Invention
The present invention relates to coating compositions which are applied to wood products in order to minimize the amount of moisture absorbed by the wood. In particular, the present invention relates to a water based sealant composition containing a polymeric binder and a particulate filler. The composition is suited particularly for application to the edges of wood products such as plywood, waferboard, oriented strand board, particle board, and the cut ends of lumber products where water absorption is a problem.
2. Description of the Prior Art
A number of water based acrylic polymer sealants are known in the art. For example, acrylic latex based sealants exhibit desirable properties such as good flexibility, good adhesion to many substrates and resistance to degradation by ultraviolet radiation. However, they have some serious weaknesses, such as poor wet adhesion and poor barrier properties against water penetration.
Others have attempted to improve the water barrier properties of such latex sealants. For example, Bullman in U.S. Pat. No. 4,340,524 introduces a hydrophobic resin in a non-gelling organic solvent, such as a vinyl chloride/vinyl acetate copolymer resin in butyl benzyl phthalate, into the acrylic latex resin in order to improve its water resistance.
Others such as Gregory in U.S. Pat. No. 4,317,755 have provided floor sealants which include a self-polishing film-forming polymer coating and an emulsified hydrocarbon solvent in an aqueous vehicle. Such floor sealants may also include waxes to improve the wear properties of the sealant.
Others such as DeRuiter et al. in U.S. Pat. No. 4,722,953 teach the use of flexible, water-resistant asphalt emulsion coatings for roof and wall surfaces, foundations, paper and cardboard packaging and lumber products. Wax is added to these coatings as an emulsifying agent and to control viscosity.
In recent years roofs in residential and other buildings have been made using sheets of plywood, waferboard, particle board and/or oriented strand board. Shingles or other types of water proof materials are then applied over the wood sheets. During construction the wood sheets may be exposed to rain before the shingles can be applied. When the wood sheets absorb water they swell. After exposure to a hard rain, the thickness of the center portions of a wafer board sheet may expand about 10% or more while the thickness of the board along its edges may expand up to about 25% or more. This is due to the fact that the edges of sheets of plywood, waferboard, oriented strand board and particle board all absorb greater amounts of moisture, and are mechanically weaker against swelling pressure, than the center portions of these same sheets. The expanded board thickness can in general create problems, which are especially serious along the joints where adjacent boards abut against one another. Edge swelling in these areas can cause a ridge to form at a joint, thus preventing shingles and other roofing materials from being evenly applied.
Wickert in U.S. Pat. No. 4,683,260 discloses a cross-linkable clear top coat for plywood and hardwood interior paneling. The coating includes an acrylic latex emulsion polymer having a glass transition temperature of about 20.degree. C. and a non-film forming polymer emulsion having a glass transition temperature of about 100.degree. C. The composition also includes a glass transition temperature of about 100.degree. C. The composition also includes a glycoluril cross-linking agent. Wickert adds Carnauba wax to make his coating buffable and silicon slip aids to make the coating shiny and smooth. The Wickert coating has not been suitable for the manufacture of commercial composite wood products because it must be cured at high temperatures and because it provides poor water barrier properties when applied in commercially acceptable amounts.
Because of the tendency for composite wood products, such as oriented strand board, to swell after absorbing moisture, cured (i.e., cross-linked) or hard (i.e., high glass transition temperature polymer) polymeric sealant coatings, such as those disclosed by Wickert, tend to crack when the wood swells due to moisture absorption. The cracking allows more moisture into the wood eventually causing even greater thickness swelling and further loss of sealant integrity.
Thus, it is an object of the present invention to provide a water based sealant composition having excellent water repellent and water barrier properties for application to wood products in order to minimize moisture absorption by the wood through the coated surface, particularly through the edges of such products. It is another object of the invention to provide a sealant coating composition which dries to form a non-cross linked coating having good flexibility in order to accommodate some swelling of the wood without cracking.